Diceware is
a method of easy-to-remember, easy-to-type, secure passphrase and
password generation. It works completely off-line and requires no
computer whatsoever, apart from retrieving the Diceware list. By
taking the passphrase generation off-line there is less room for
mistakes to be made.

The reason these password are easy to remember is that they are simply
a series of words in your native language. This also tends makes them
easier to type without lots of practice as you should already be used
to typing words.

The lists are cryptographically signed by Arnold G. Reinhold so you
can verify that I have not tampered with it. I must also note that,
unfortunately, the author requires that this list only be distributed
non-commercially, which limits its usefulness but allows me to
distribute it here.

I also came across another list called DialDice, which I have
mirrored here and signed with my own key,

Diceware works by rolling five 6-sided dice (or rolling one 6-sided
die five times, etc.) and using the result to look up the word in one
of the above lists of 6^5, or 7776, words. Each word is worth about
12.9 bits of entropy,

log2(7776) =~ 12.9248

So if you want a password worth about 40 bits -- which is about 7
letters of a random alphanumeric password -- you would generate three
Diceware words. They can be concatenated in any order and in any
fashion. When I use Diceware, I just mash them together, like
"lancealertgrow". Note that the space bar makes a distinctive sound
when pressed, so if you put spaces between your words a listener will
be able to tell how many words you use.

If you don't like what you rolled the first time, DO NOT
generate a new one as an attempt to get someting "better". If you do
this, you will greatly weaken your passwords because you are selecting
passwords from a much smaller pool of possibilities (a very small pool
that contains only passwords you like).

The number of possible three-word passwords is 470,184,984,576. That's
right: 470 billion. Because you are selecting passwords with
dice, each password is as equally likely as the next. Even if an
attacker knew you used Diceware and knew what word list you used, that
still leaves a handful of guesses out of that 470 billion
possibilities.

At first it may be confusing, but it actually doesn't matter what the
words are or how long they are. It doesn't matter that there are no
capitals or special characters. It is the simple fact that there are
7776 words, and one was selected three times.

7776 * 7776 * 7776 = 470184984576

The Diceware website goes into a bit more detail on this.

If the computer system you use annoyingly requires passwords to
contain special characters (which is done to increase entropy in
passwords that actually are poor), Diceware also provides a
method of adding some of those, which adds a couple more bits worth of
entropy. If you don't care about those extra bits, you can throw your
own in.

For passphrases,
Diceware recommends 6 words, about 77.5 bits, which it claims
should be out of range of brute-force attacks from anyone for at least
the next 20 years. If you really think you need more than 6 words, you
should consider hiring guards for all your computer equipment.

I was working on my own Diceware word list to release into the public
domain. The purpose was to provide a word list without any
distribution restrictions, unlike all the Diceware lists I have
found. But making word lists is hard! I wrote a number a little
filters -- word length, no sub-words, spell checking, no special
characters, etc -- to pull out good words from a large word list, then
used some sample English text from Wikipedia to get some frequency
information so that I am was selecting common words over less common
words. I still need to go over the final list by hand to make sure it
all looks good. This is a long tedious process. Carefully examining
7776 words is quite a lot of work. Someday I will finish it.

In the event that you don't have any dice available, or you want to be
able to generate Diceware passwords on the fly automatically, I have
written a little program that will use /dev/random,
assuming there is a true RNG behind it, to roll virtual dice. It can
use a Diceware word list or your local dictionary word list which
contains many more words (usually found at
/usr/share/dict/words). Grab it here,

Chris Wellons

Born and raised in Pittsburgh, I am a happily married
computer engineer living and working in Maryland. This is a
blog about hobby programming. To explore this site, have a
look at the list of topics
or the archives.